The purpose of this study is to measure cerebral blood flow with positron emmision tomography (PET) and H2150 during a short-term (declarative) verbal memory task in patients with posttaumatic stress disorder (PTSD) and healthy subjects. We hypothesize that PTSD patients will have a blunted hippocampal blood flow response to short-term verbal (declarative) memory tasks relative to controls. Preliminary data shows a failure of hippocampal activation in PTSD.